Evaporative heat exchangers are well known in the art and are particularly utilized in a variety of commercial and industrial applications. Some users of evaporative heat exchangers demand certain performance requirements. For instance, one user might require that the outlet temperature of the process fluid flowing out of the evaporative heat exchanger be, say, 87° Fahrenheit plus or minus, say, 0.75° Fahrenheit. Thus, to be competitive, manufacturers of evaporative heat exchangers must provide a way to control and maintain the outlet temperature of the process fluid as required by the user. One way to control and maintain the outlet temperature of a conventional heat exchanger is disclosed in U.S. Pat. No. 7,310,958 to Carter et al.
The Carter patent teaches a method of controlling the operation of a heat exchanger that has a dry, non-evaporative section and an evaporative section. The method includes the steps of sensing whether a water spray is on in the evaporative section, establishing that the water spray is on and comparing fan output to a predetermined energy switch fan output. If the fan output exceeds the predetermined energy switch fan output, the method compares the outlet fluid temperature to a predetermined set point temperature and reduces fan output if the outlet fluid temperature is less than the set point temperature. The method also senses whether the water spray is on and establishes that the water spray is on, compares fan output to a predetermined energy switch fan output and sets the fan speed to about equal the energy switch fan output, compares the outlet fluid temperature to a predetermined set point temperature and reduces fluid flow to the evaporative section if the outlet fluid temperature is less than the set point temperature.